Motor control apparatuses, for example, electric power steering (EPS) apparatus disclosed in U.S. Pat. No. 6,497,303 (JP-A-2002-12157) or JP-A-2003-2222 uses feedback control to make a motor current converge to a desired current value (i.e., torque command). In a small current range such that the desired value is small, the feedback control is generally required to produce the motor current precisely in accordance with a small change in the desired value. In contrast, in a large current range such that the desired value is large, the feedback control may not be required to produce the motor current precisely in accordance with the small change in the desired value, because the absolute amount of a motor torque produced in the range is large.
Based on this requirement, a current detection circuit having switchable sensitivity has been proposed. The sensitivity switchable detection circuit detects a current with high accuracy in the small current range and with relatively low accuracy in the large current range, thus having a wide dynamic range. The sensitivity switchable detection circuit allows a current detection apparatus having high accuracy to be provided at low cost. Further, the sensitivity switchable detection circuit allows a communication bit rate between the sensitivity switchable detection apparatus and a signal processing device (e.g., microcomputer) to be reduced, thereby simplifying the signal processing operation performed by the signal processing device.
The sensitivity switchable detection circuit can be modified to detect various input quantities including the motor current and has advantage in terms of cost performance.
The sensitivity switchable detection circuit has a high gain region where a high amplification of an input quantity (e.g., electric current) is performed and a low gain region where a low amplification of the input quantity is performed. When the input quantity is small, the detection circuit uses the high gain region. In contrast, when the input quantity is large, the detection circuit uses the low gain region.
However, when the two gain regions are switched, a considerable change in a detected signal occurs between before and after the region switching operation, due to gain errors of amplifiers, circuit errors, or the like. Specifically, the detected signal shows step changes, i.e., sudden changes, even when the input quantity varies only a little between before and after the region switching operation.
If an output signal of a motor control apparatus suddenly changes, a motor torque suddenly changes. In the EPS control, the sudden change of the motor torque causes uncomfortable feelings in steering, for example.
Taking measures such as reducing differences in manufacturing tolerances and temperature variations between the high gain region and the low gain region of the detection circuit may prevent the above problem. However, such measures cause a reduction in yield and an increase in parts cost of the detection circuit, and therefore are not practical in most applications.